Line printers are typically comprised of a print head assembly slidable along a guide means and normally moving in a forward or print direction from the left-hand to the right-hand margin of a paper document, at which time printing occurs. As soon as the print head reaches the right-hand margin or as soon as a short line or print, i.e., shorter than a full line of print, is completed, the drive mechanism is typically released to place the print head under the influence of a return spring or clutch actuated mechanism which rapidly returns the print head, in what is typically referred to as a carriage return operation, toward the left-hand margin. The print head must be brought to an abrupt stop preparatory to printing the next line of characters. To accomplish this, it is typical to provide a rubber bumper mounted to the carriage assembly which abuts the surface of a stationary member attached to the printer frame or a surface of the frame itself. Since the print head is caused to impact the stationary surface at full velocity, the assembly is jarred and may be damaged as a result of performance of many "carriage return" operations.
In order to prevent jarring of the components, a dash-pot assembly is typically employed and comprises a stationary mounted open-ended cylinder and a co-axially aligned piston or plunger secured to the print head assembly. As the print head moves rapidly in the "carriage return" direction, the plunger enters into the open end of the cylinder to compress the air captured therein, some of which is released through a breather hole at a controlled rate, thereby bringing the print head to a stop without jarring action.
Immediately thereafter, the print head is moved in the print direction, causing a plunger to be withdrawn from the cylinder. The vacuum condition created therein acts as a deterrent, preventing the print head from being rapidly accelerated to "print" velocity.